1. Field of the Invention
The present invention relates to a biasing circuit for a field effect transistor operating at ultra high frequencies. With this biasing circuit, the biasing voltages on two electrodes on the transistor, one of which is the gate, can be varied simultaneously. It is particularly interesting in the case where it is itself integrated on a monolithic integrated circuit comprising other functions, for it occupies a much smaller area on the semi conductor material chip than known circuits.
2. Description of the Prior Art
So as not to disturb the microwave--or ultra high frequency--signals, the biasing circuits must be filtered by means which let the DC biasing currents or voltages pass but isolate the microwave power. These means are most often chokes, microband lines or very high value resistances. The drawback with the first two is that they do not have the dimensions and technology compatible with integrated circuits. Resistances have the drawback that, if a drain current is required, the voltage drop through the resistance is high, and the biasing voltage must be high.
In other known circuits, the drain bias of the main field effect transistor is provided by a secondary field effect transistor, current source connected with the gate connected to the source: but the bias voltage of the drain is fixed in this case and it is not possible to vary the gate bias voltage, so the biasing point of the main transistor.